Nowadays, electroluminescent displays such as organic light-emitting diode (OLED) displays are usually driven in either a passive matrix (PM) mode or an active matrix (AM) mode. An OLED display driven in the PM mode (named PMOLED) uses external driver circuitry that performs a voltage scanning of the display panel line by line alternately: that is, each line of pixels are lit alternately in sequence. Structurally, a PMOLED display panel consists of a set of electrodes that are arranged in the longitude direction (row electrodes) and a set of electrodes that are arranged in the latitude direction (column electrodes), wherein the electroluminescent organic layer is sandwiched in between the row and column electrodes (FIG. 1A). As shown in FIG. 1A, a PMOLED display panel (1000) consists of a substrate (1001), column electrodes (1002), the organic layer (1003), and row electrodes (1004). Generally, the substrate (1001) consists of a transparent material. The column electrode (1002) is usually the positive electrode, which consists of a transparent metal oxide layer such as indium tin oxide (ITO). The organic layer (1003) usually consists of multiple layers of functional organic materials, such as the hole injection (HI) layer, hole transporting (HT) layer, electron blocking (EB) layer, emissive (EM) layer, hole blocking (HB) layer, electron transporting (ET) layer, and electron injection (EI) layer, etc. The selected organic materials can be either polymeric materials or small molecule organic materials. In a tandem OLED, there are additional charge separation layers in between each individual OLED stack. The row electrode (1004) is usually the negative electrode and it is the common electrode for the same row the pixels. The pixel brightness is determined by the voltages applied to the column electrodes, V1, V2, V3 . . . Vm (refer to FIG. 1A). The row electrodes are generally aluminum electrodes having a thickness of approximately 150 nm.
In this application, the meaning of the terminology “organic light-emitting diode” or “OLED” includes small molecule OLED, polymer OLED, and tandem OLED; the terminology “row electrode” refers to the electrode that is common to all pixels on the same electrode, and it can be either the positive electrode or the negative electrode; when the voltage value of a column electrode is addressed, it should be understood as the absolute value of the voltage.
FIG. 1B depicts the waveform of the scan-voltage pulses used in a PMOLED. In this driving mode, each row of pixels is switched on once within each scanning cycle, and the “switched-on time” of a pixel is inversely proportional to the number of row electrodes. Since the pixel's “switched-on time” is short, it is required that the pixel's brightness at the moment when it is switched on to be very high, in order to achieve a reasonable apparent brightness. For example, if a display panel of n rows is scanned with a perfect rectangular voltage waveforms at a frequency of z (Hz), the length of the scanning cycle (τ) is τ=1/z (second); within each scanning cycle, the “switched-on time” for each row of pixels (Δto) is approximatelyΔto=τ/n=1/(nz)second,  (1)whereas, the pixel's “switched-off time” is τ(n−1)/n (second). In other words, there is only 1/n of the time that a pixel is switched on and there is (n−1)/n of the time the pixel is switched off. If the average (apparent) brightness of a panel is Bo, the pixel brightness during its switched-on period (Bp) should be Bp=Bo×n.
Therefore, when n is significantly large, the pixel brightness BP is much greater than the apparent brightness of the panel. This has resulted in a series of negative consequence, including reduced lifetime of the device, reduced electricity-to-light conversion efficiency of the device (the electricity-to-light conversion efficiency of most OLEDs decrease significantly at high brightness), and increased power consumption on the electrodes due to the increase of driving current. In addition, because an OLED device possesses a capacitance, when the scanning frequency is sufficiently high, the charge/discharge current may become significant that further increases the load on the driver circuitry and the power consumption on the electrodes, and reduces the power usage efficiency.
Limited by the maximum brightness that an OLED device can achieve and the conductivity of the electrodes, the size of a PMOLED display is generally no more than 100 lines. Therefore, PMOLEDs are only adapted for low-resolution and small-size (approximately 1 inch) displays. Although the multi-line addressing technology recently developed by DialogSemiconductor has boosted the performance of PMOLED in certain extent—240 lines, equivalent to ¼ VGA has been achieved—currently the resolution of a single PMOLED could hardly exceed the level of quarter VGA. For higher resolution (i.e. over 1000 lines) OLED displays (such as high-definition television, HDTV), the existing PMOLED approach cannot be used directly (unless tiling multiple PMOLEDs to form a larger display).
The AM driving mode uses an internal thin-film transistor (TFT) driver circuitry to control the brightness of the individual pixel independently, i.e. each pixel is controlled by a dedicated TFT driver built into the display backplane. The pixel brightness is determined by the amount of current delivered from the TFT driver (determined by the switched-on time, in case drive-current is constant). FIG. 2A illustrates the layout of an AMOLED, where the AMOLED display (2000) consists of a substrate (2001), an array of pixels consisting of anode (2002) and TFT driver (2003), the organic layer (2004), and cathode (2005). Wherein, the anode (2002) of each pixel is electrically connected to the output of the corresponding TFT driver, and cathode is the common electrode. In a “bottom-emitting” AMOLED, the anode is usually ITO and the cathode commonly uses aluminum or silver. In a “top-emitting” AMOLED, 2002 is a metal with high work-function and cathode 2005 is the transparent electrode.
FIG. 2B illustrates an example of a TFT driver circuitry used in an AMOLED. In this design, a pixel is controlled by a circuitry consisting of 4 TFTs (T1, T2, T3, and T4). Wherein, VDD is the positive pole of the power source, GND is the ground (the negative pole of power source), “Address line” is the driving line, and “Data line” is the data line.
The advantage of AMOLED is that its pixel does not require to work at a very high brightness, and therefore has longer lifetime and higher efficiency than PMOLED. As a result, it is currently adapted by most OLED displays. Theoretically, AMOLED can be used for high-resolution and large-screen displays. In practice, however, due to the huge amount of TFTs required, the complexity of the processes used to produce the TFT-backplane, the low yield, and limited by the material, currently only small-size (10-inch or below) AMOLEDs can be mass-produced. So far, none of the attempts in commercializing the mid-size and large-size (such as computer screens and large screen TVs) AMOLED are successful, due to the low manufacturing yield of the TFT backplane and the high production cost. On the other hand, the TFT driver circuitry itself consumes a significant amount of energy, which not only reduces the power usage efficiency but also, if used for a long period of time, produces a significant amount of heat that may raise the temperature and affects the lifetime of the device.
Additional technical details in regards to the driving method and construction of PMOLED and AMOLED displays can be found in US patents US20060091794, US20070114522, US20070152923, US20100085280, U.S. Pat. No. 5,952,789, U.S. Pat. No. 7,847,763, US 20130257845, US20130235023, and European patent EP2461311.
In an attempt to overcome the problems that PMOLED is incapable for making high-resolution and large-size panels while AMOLED is too expensive to produce, the US patent US2007/0001936A1 proposed a capacitor driving method that is different from either the conventional PM or AM mode. However, as described in US patent US2007/0001936A1, the electrodes used are basically the same as those in a conventional PMOLED. As a result, although US patent US2007/0001936A1 may be able to improve the efficiency and performance of existing PMOLED displays in a certain extent, the conductivity of the electrodes is still low and therefore cannot be practically adapted by the industry. In other words, the problem of electrodes having limited conductivity still exists in US patent US2007/0001936A1, and therefore it cannot fundamentally resolve the problems preventing PMOLED to be made high-resolution and large-size. Furthermore, there are several additional problems exist in the capacitor-drive technology described in US patent US2007/0001936A1, such as the circuit is too complicated, the manufacturing process too complex, production cost too high, and the OLED panel thickness increased, etc.